Image guided surgery is becoming more common, especially in the areas of inter-cranial surgery. Systems are utilized to take data gathered from pre-operative scans by MRI, CT scanners, ultrasounds, or the like. The data is used to generate a three-dimensional image to guide a surgeon during an operation. Often this includes some method for tracking an instrument location with respect to the image displayed by the system. Generally, the image is registered relative to locators attached to the patient. Then, the position and orientation of the surgical instruments is registered and tracked relative to the image and the patient so that the location and orientation of the instruments relative to the image is continuously displayed while the surgeon performs the surgery.
The problem with using the pre-operative image is that the object selected may have shifted between the time the pre-operative image was taken and the time of surgery. This is especially true once surgery has begun and the shape of the intercranial cavity changes as the surgeon gains access. Changes in the pre-operative image and the actual surgical subject introduce variations into the surgical process. In matters like intercranial surgery the tolerance for variations is low, thus even small changes between the image and actual subject may cause problems and make the surgery less effective. To solve this problem additional images may be taken during surgery to update the previously received information. However, selecting the area to be scanned, setting up the intra-surgery scanner, and performing the scan require movement of bulky equipment and surgery must be stopped to set up the equipment properly and perform the scan. In addition it is difficult to move the equipment to the desired area to be scanned thus increasing the time and effort required to update the image properly. Thus, because of the time and effort required, intraoperative scans are generally not used.